The present invention refers to the field of optical devices for the spectral range of extreme ultraviolet (EUV) and/or soft X-ray radiation comprising optical components with reflective surfaces for reflecting the EUV and/or soft X-ray radiation. Such optical devices are required, for example, for EUV lithography, in which grazing incidence mirrors and/or multilayer mirrors are arranged in a vacuum chamber between the radiation source and the wafer substrate to be irradiated. Typical materials used for the reflecting surfaces of the grazing incidence mirrors are for example ruthenium (Ru), palladium (Pd) or molybdenum (Mo). Multilayer mirrors for the above spectral range, which are suited for vertical or near vertical incidence, typically comprise a combination of layers of molybdenum and silicon (Si). Often also a top layer of ruthenium is applied for protecting the underlying layers.
A problem mainly arising during operation of optical devices with such reflecting optical components is the decrease of reflectivity over time. This reduction in reflectivity can be caused by contaminations of the reflecting surfaces due to debris from the radiation source or to reactions with gas remaining in the vacuum chamber during operation. Radiation sources for EUV lithography today are gas discharge plasmas or laser plasmas. The substances used for plasma generation, however, can move from the radiation source to the optical components and condense on the optical surfaces, thereby reducing their reflectivity. The material released from the radiation source and moving in the direction of the optical components is called debris. Other contaminations of the optical components can result from the fabrication process, transport or mounting of the optical components. Furthermore, the reflectivity of the reflecting surfaces can be reduced by an increased surface roughness, a reduced density or a reduced thickness of the reflecting layer or layers, due to the operation of the radiation source.
WO 2004/092693 A2 discloses a method and apparatus for debris removal from a reflecting surface of an EUV collector in an EUV lamp. In this method, a controlled sputtering ion source is created which comprises a gas with the atoms of the sputtering ion material and a stimulating mechanism causing the atoms of the sputtering ion material to exit in an ionized state. With this sputtering ion source the debris material deposited on the reflecting surfaces of the EUV collector is removed by sputtering. In order to avoid a removal of the top layer of the reflecting surface, the ionized state of the sputtering ion material is selected to have a distribution around a selected energy peak that has a high probability of sputtering the debris material and a very low probability of sputtering the material of the top layer of the reflecting surface.